Electric heating unit



y 1934- w. D; KELLY ET AL 1,960,221

ELECTRIC HEATING UNIT Original Filed April 15, 1930 William D. Kelly, Claude C. Rhodes,

Then Attorneg.

Patented May 22, 1934 UNITED STATES PATENT OFFICE ELECTRIC HEATING UNIT Application April 15, 1930, Serial No. 444,566 Renewed July 22, 1933 14 Claims.

Our invention relates to electric heating units, more particularly to electric heating units of the sheath wire type, and has for its object the provision of an improved method of making an electric heating unit of this character.

More specifically our invention relates to electric heating units of the sheath wire type in which a resistance element is embedded in a compacted mass of powdered insulating material enclosed by a metallic sheath, such for example, as described and claimed in the United States patent granted to C. C. Abbott, No. 1,367,341, dated February 1, 1921. Our invention is especially directed to relatively long electric heating units of this type.

It has been very difiicult, heretofore, to make a sheath wire heating unit of relatively great length because of the tendency for the resistance element and the terminals to move from their respective central positions in the metallic sheath while the powdered insulating material is being loaded and while it is being compacted. As a result of this tendency, the clearance between the resistance element and the sheath frequently was reduced, at times to such a degree that the resistance element grounded.

In carrying our invention into effect in one form thereof, we distort or shape the metallic sheath so as to provide a plurality of reduced portions spaced at intervals along the length of the sheath and positioned to positively hold the resistance element together with its terminals centrally of the sheath. After the resistance ele ment and insulating material have been inserted in the sheath, we reshape the sheath so as to remove the reduced-portions.

For a more complete understanding of our invention reference should be had to the accompanying drawing in which Fig.1 is a broken sectional View illustrating an electric heatingunit constructed in accordance with our invention before the metallic sheath is shaped so as to provide the reduced portions; Fig. 2 is an end elevation of Fig. 1; Fig. 3 is a sectional View illustrating the sheath after it has been shaped so as to provide the reduced portions; Fig. dis a sectional view taken through the line l% of Fig. 3; Fig. 5 is a sectional view illustrating the assembled heating unit after the loading but before the 50 swaging operation; Fig. 6 is an elevation partly in section of the heating unit after the sheath has been reshaped so as to give the sheath a substantially uniform diameter throughout its length; Fig. 7 is a fragmentary view illustrating the metallic sheath after it has been. shaped so as to form the reduced portions by another method in accordance with our invention; and Fig. 8 is a View similar to Fig. 7 but illustrating the sheath shaped by still another method in accordance with our invention.

Referring to the drawing, in one form of our invention the elongated heating unit 10 comprises a helical or sinuous resistance element 11 extending centrally of a metallic sheath 12. Preferably, the resistance element will be made of a suitable material having considerable resiliency such as nickel-chromium alloy. The heating element in the finished electric heating unit (Fig. '6) is embedded in a compacted mass of powdered heat refractory insulating material 13, such as magneslum oxide. Metallic terminals 14 and 15 are provided for the resistance element 11.

As has been pointed out, we provide the metallic sheath with a plurality of reduced or fiattoned portions 12a positioned so as to maintain the heating element and the terminals centrally of the sheath during the insulating material loading and compacting operations. Generally the heating element will be assembled with the sheath before the reduced portions are formed.

In forming the heating unit in accordance with our invention first one of the terminals, as for example the terminal 14, will be assembled and secured to the resistance element. Preferably, the terminal will be secured in a manner substantially the same as'that described and claimed in the United States Patent granted to C. C. Abbott No. 1,494,938, dated May 20, 1924. In accordance with the method there described, the terminal is provided with a threaded portion 16 on which the end portion of the resistance ele ment is turned or screwed. It will be understood that the diameter of the helix is slightly less than the diameter of the terminal at the roots of the threads so that the helix is expanded some- What when it is turned on. The resistance conductor consequently contracts by reason of its resiliency on the threaded portion thereby making a good electrical and mechanical connection with the terminal.

The metallic sheath 12 is prepared for the assembly of the heating element 11 by providing its ends with counterbores l7 and by "ioroughly cleaning the interior of the sheath. The re sistance element 11 with its terminal 14 attached 5 is then inserted lengthwise within the sheath as shown in Fig. 1. The sheath isclosed at one end, for example, at the left hand end by a solid washer 18 which is suitably secured to the outer end of the terminal 1 1 and which is received in 110 the counterbore 1'7 at the left end so as to hold the terminal centrally of the sheath. The terminal is then secured to its end of the heating element. This may be accomplished by drawing the end portion of the heating element out of the sheath far enough to permit thc'terminal 15 to be screwed into it in a manner similar to that described in connection with the assembly of the terminal 14. Thus, the terminal 15 will be provided with a threaded portion 16 for re ceiving its end portion of the heating element. The terminal 15 is secured centrally of the sheath and against longitudinal movement under the tension of the resistance element by a star washer which is received in the right hand counterbore 17. It will be understood that the heating element will be placed under a tension sufficient to separate its turns and hold it central within the sheath and also to prevent its kinking while the reduced portions are being formed in the sheath.

After the heating element has been assembled with the sheath as shown in Fig. l, the sheath is ready for the shaping operation. As shown (Figs. 3 and 4), the sheath is shaped so as to provide the reduced portions 12a spaced at intervals along the length of the sheath. Preferably and as shown, the reduced portions will be provided in pairs so that the portions forming each pair will be located comparatively close to each other while the pairs will be spaced at comparatively great intervals. However, these intervals 'will not be great enough to permit local displacement of the resistance element.

It is also preferable to position the portions forming each pair substantially at right angles to each other. By reason of this arrangement, the reduced portions of each pair taken together form a square-shaped retaining section. It will be observed by reference to Fig. 4 that the passageway through the sheath at these reduced portions is just large enough to receive the ter minals and the heating element 11 and thus by reason of the fact that the portions of each pair are arranged substantially at right angles to each other, the heating element and terminals will be secured centrally of the sheath by a plurality of retaining sections spaced at intervals along its length. It will also be observed that the flattened portions of each pair are relatively wide so as to provide a free passageway through the sheath. It will be understood that the reduced portions'may be formed by any suitable means, as for instance, by a press.

The sheath is now ready for the loading operation. The sheath is filled through the openings provided by the star loading washer 20 with the powdered insulating material 13. To accomplish this the sheath is held in an upright position in a special vibrating machine (not shown) with the end provided with the star washer uppermost. As the sheath is filled with the insulating material through the openings provided by the star washer, it is vibrated to shake the insulating material together and to distribute it uniformly around the terminals and the resistance element. It will be understood that the powdered insulating material will readily pass through the reduced sections from the upper portions of the sheath into the lower portions thereof. When the sheath has been filled, a solid washer 21 is inserted in the right hand counterbore 17 over the star washer. Both ends of the sheath are now pinched or bent at intervals 22,

as shown in Fig. 5, so as to secure the washers in place.

The entire unit is now passed through a swaging or rolling machine for the purpose of compacting the insulating material within the sheath to a hard dense mass, the swaging operation reducing the diameter and increasing the length of the sheath. The first pass or two through the swaging machine generally will remove the flattened portions and the sheath will assume a complete cylindrical shape throughout its length. It will be observed, therefore, that during the initial swaging operation at which time the tendency for the insulating material to pile up or flow ahead of the swaging machine dies is the greatest, the resistance element and terminals will be held in their respective central positions by means of the flattened portions provided in the sheath. It will be understood that the insulating material will be compacted sufficiently by the first swaging operation to hold the element and its terminals firmly in place during the final swaging operations.

In case it is necessary to reduce the diameter of the unit considerably, it may be desirable to pass the unit through the swaging machine a number of times so as to reduced its diameter gradually. In this event the unit may be an nealed between the successive swaging operations to prevent the resistance element and likewise the sheath from becoming too hard and brittle.

After the unit has been swaged to the desired extent, the washers 18 and 20 are cut off or otherwise removed and the ends of the unit squared oil as shown in Fig. 6. The terminals are held firmly in place by the compacted insulating material in which they are embedded.

The metallic sheath may be shaped so as to form the reduced portions by indenting it as shown in Fig. '7. The sheath of Fig. '7 is provided with a plurality of single crimps or indents 30a progressing from one end of the sheath to the other and each succeeding indent being positioned approximately 90 about the sheath from the preceding indent. These crimps preferably will be positioned so close to each other that each one will merge with the next and thus all of the crimps taken together will approximate a continuous helical flattened portion. This arrangement provides a substantially square retaining section for the resistance element. Preferably and as shown, each of the terminals 31 will be secured centrally of the sheath by a pair of opposed crimps 31b arranged as are the terminal retaining crimps 12 of Fig. 3. It will be understood that after these crimps have been formed the method of completing the heating unit will be identical with that described in con nection with Figs. 1 to 6 inclusive.

The sheath may also be distorted by providing in it a continuous substantially helical flat portion 32 as shown in Fig. 8. It will be understood that the sheath when distorted by this method will provide a substantially circular retaining section for the resistance element.

While we have shown certain specific embodiments of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made, and we, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The method of making an electric heating unit which comprises mounting a resistance element lengthwise in a sheath, giving said sheath a shape so as to provide a plurality of flattened portions justlarge enough to receive said resistance element and arranged to hold positioned said resistance element in spaced relation with respect to the inner Walls of said sheath, filling said sheath with insulating material and then reshaping said sheath so as to remove said flattened portions.

2. The step in the method of making an electric resistance unit having a resistance element embedded in an insulating material and closed in a metallic sheath which consists in securing an electrical resistance element centrally or" an elongated metallic sheath by giving said sheath a shape to provide a plurality of portions reduced in cross section arranged to define retaining sections in said sheath of approximately the size of said resistance element so as to prevent displacement of said resistance element from its central position.

3. The methodof making an electric heating unit which comprises mounting a resistance element centrally of a metallic sheath, indenting said sheath so as to provide at intervals along its length flattened portions elongated transversely of said sheath-and arranged to define retaining sections of such depth that they will engage said resistance element in the event it tends to move laterally from its position central of said sheath whereby said resistance element is maintained centrally with respect to said sheath, filling said sheath with powdered heat refractory insulating material, reshaping said sheath so as to remove said flattened portions and compacting said insulating material to a hard dense mass around the resistance element.

4. The method of making an electric heating unit which comprises mounting a resistance element lengthwise in a sheath, giving said sheath a shape so as to provide a plurality of flattened portions spaced at intervals along the length of said sheath arranged to define a plurality of retaining sections in said sheath just large enough to receive said resistance element and thereby maintain said resistance element in spaced relation with respect to the inner walls of said sheath, filling said sheath with powdered insulating material and then swaging said sheath so as to remove said flattened portions and to compact said insulating material about said resistance element.

5. The method of making an electric heating unit having a resistance element mounted in a sheath which comprises providing indents in said sheath of such depth that they define reduced sections having approximately the size of said resistance element and thereby prevent relative lateral displacement between said resistance element and said sheath, filling said sheath with powdered insulating material and then reshaping said sheath to remove said indents and to compact said insulating material about said resistance element.

6. The method of making an electric heating unit'which comprises mounting a resistance element centrally of an elongated metallic sheath with the element under tension, then indenting said sheath at a plurality of points so as to provide flattened portions spaced at intervals along the length of said sheath said flattened portions being positioned in pairs in said sheath with the flattened portions of each pair arranged at an angle to each other, whereby said resistance element is maintained centrally of said sheath, filling said sheath with powdered heat refractory insulating material and then reducing said sheath so as to remove said flattened portions and to compact said insulating material to a hard dense mass around the heating element.

'7. The method of forming an elect 'c heating unit which comprises mounting a resistance element together with terminal members centrally of a metallic sheath, giving said metallic sheath a shape so as to provide a plurality of portions reduced in cross section arranged to define re- I cluced retaining sections in said sheath just large enough to receive said resistance element and said terminals and thereby prevent lateral displacement of said resistance element and its terminals, filling said sheath with powdered insulating material, and then reshaping said sheath so as to remove said reduced portions and to compact said insulating material about the terminals and resistance element.

8. The method of making an electric heating unit which comprises mounting a resistance element with terminal members lengthwise in and centrally of a metallic sheath with the element under tension, indenting said sheath so as to provide a plurality of flattened portions spaced at intervals along the length of said sheath, said flattened portions being positioned in pairs in said sheath so that the flattened portions of each 105 pair are substantially at right angles to each other whereby said resistance element and said terminals are maintained centrally of said sheath, filling said sheath with powdered heat refractory insulating material and then reshaping said sheath so as to remove said flattened portions and to compact said insulating material about the terminals and the resistance element by reducing said sheath.

9. The step in the method of making an electric resistance unit having a resistance element embedded in an insulating material enclosed in a metallic sheath which consists in securing an electrical resistance element centrally of an elongated metallic sheath by distorting said sheath so as to provide a plurality of portions reduced in cross section and arranged to define retaining portions in said sheath just large enough to receive said resistance conductor and thereby prevent displacement of said resistance element from its central position.

10. The step in the method of making an electric heating unit having a resistance element embedded in an insulating material enclosed in a metallic sheath which consists in securing an electrical resistance element and its terminals within an elongated metallic sheath by extruding said sheath so as to provide a plurality of portions reduced in cross section and positioned to prevent lateral movement of said resistance element and its terminals relative to said sheath.

11. The method of making an electrical heating unit which consists in introducing an electrical resistance element into a sheath, giving said sheath a shape so as to provide a plurality of pairs 'or". co-operating flat elongated reduced portions in said sheath at intervals and defining retaining sections arranged to prevent lateral displacement of said resistance element,

filling said sheath with electrically insulating 145.

material and then reshaping said sheath so as to remove said reduced portions.

12. The method of making an electric heating unit which comprises mounting a resistance element lengthwise in and substantially centrally of a metallic sheath, indenting said sheath so as to provide a plurality of flattened portions spaced at intervals along the length of said sheath, said flattened portions progressing lengthwise of said sheath and each succeeding indent being positioned approximately about said sheath from the preceding indent whereby said resistance element is maintained substantially central- 1y of said sheath, filling said sheath with powdered heat refractory insulating material and then reshaping said sheath so as to remove said flattened portions and to compact said insulating material about said resistance element.

13. The method of making an electric resistance unit which comprises mounting a resistance element substantially centrally of a sheath, securing said resistance element substantially in its central position by providing in the walls oi said sheath a substantially continuous helical flat porfilling said sheath with powdered heat refractory insulating material and then swaging said sheath so as to remove said helical fiat portion and to compact said insulating material about said resistance element.

' WILLIAM D. KELLY.

CLAUDE C. RHODES. 

